Measurement of the CKM matrix element |Vcb| using the decay B → D∗lνl at Belle II

Sumitted to PubDB: 2025-05-02

Category: Phd Thesis

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Principal Authors Christoph Schwanda
Non-Belle II authors Daniel Dorner
Date 2024-08-06
Belle II Number BELLE2-PTHESIS-2025-011
Abstract In the standard model of particle physics the experimentally observed quark mixing is described by Cabibbo-Kobayashi-Maskawa matrix. The transition rate from bottom to charm quarks is proportional to the matrix element magnitude |Vcb|. Additionally |Vcb| is one of the fundamental parameters of the standard model, i.e. it can not be determined by theory alone. This thesis aims to extract |Vcb| from data obtained by the Belle II experiment at the SuperKEKB collider in Tsukuba, Japan, which collides electrons and positrons at a center of mass energy of 10.58 GeV and started taking data in 2019. The data sample size used for this study is equivalent to an integrated luminosity of 364 fb−1. At the time of writing determining |Vcb| through semileptonic B → Xclνl decays is the favored approach. The B corresponds to a B meson, Xc a hadronic system containing a charm quark and l is a charged, light lepton, i.e. either an electron or muon with its associated neutrino νl. These measurements can be done using two different approaches, namely inclusive and exclusive. For an inclusive measurement all B → Xclνl final states are reconstructed, while exclusive measurements investigate a single decay chain, e.g. B0 → D∗+l−νl. While both approaches use different theoretical inputs their results should be the same. However, there is a long standing 3.3σ tension between inclusive and exclusive measurements, which is the main motivation behind this thesis. In this thesis the extraction of |Vcb| is studied by reconstructing B0 → D∗+l−νl decays through the subsequent decays D∗+ → D0π+ and D0 → K−π+. By extracting the amount of signal candidates from the sample the partial decay rates ∆Γ/∆w∆cosθl in bins of the hadronic recoil w and the angular kinematic variable cosθl can be determined. These partial decay rates are related to |Vcb| and these relations can be described via two different parameterizations, i.e. the Boyd-Grinstein-Lebed and Caprini-Lellouch-Neubert parameterizations. The matrix element |Vcb| and the form factors can be measured for both parameterizations via a numerical fit to the decay rates. Through this analysis strategy we measure |Vcb|BGL = (38.100±0.194±0.754±0.674)×10−3 and |Vcb|CLN = (38.582 ± 0.247 ± 0.844 ± 0.560) × 10−3, for the Boyd-Grinstein-Lebed and Caprini-Lellouch-Neubert parameterizations, respectively from simulations with a competitive precision. The provided uncertainties in order of appearance are statistical, systematic and theoretical.

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